1. Field of the Invention
The present invention described herein relate generally to a twin-screw extruder which kneads materials such as resin raw materials and is especially used for kneading resin raw materials and powdery fillers, and a kneading method using the twin-screw extruder.
2. Description of the Related Art
A twin-screw extruder which kneads materials such as resin raw materials in pellet or powdery form has been known. In general, the twin-screw extruder comprises a material feed opening which feeds materials, insertion holes through which the materials fed from the material feed opening pass, and a barrel comprising a discharge opening which discharges the materials kneaded during the passage through the insertion holes. Two screws are inserted into the insertion holes of the barrel. The two screws are provided parallel to each other, maintaining a certain distance between them. The two screws rotate in the same direction while they engage with each other.
For example, Patent Literature 1 discloses a twin-screw extruder comprising the above barrel and two screws. Its performance is high in processing materials. Thus, the residence time of materials in the barrel is shortened, and the physical properties of kneaded materials are not degraded. For example, Patent Literature 2 discloses a twin-screw extruder comprising the above barrel and two screws. The twin-screw extruder further comprises an air vent device to increase the production capacity.
As the screw provided in the above twin-screw extruders, a sectional screw whose shape (structure) can be changed is employed to knead various materials. The sectional screw is composed by fitting a plurality of screw elements or kneading discs having the same external diameter into the external circumference of the screw shaft. The shape (structure) of the sectional screw composed in this manner is suitable for the materials to be kneaded.
In the conventional technique, twin-screw extruders have been improved as follows. A deep-groove screw of a double thread screw has been employed to increase the amount of materials to be processed per unit time. The kneading volume of the screw has been increased by reducing the root diameter of the screw relative to the external diameter of the screw. In addition, the rotary driving force of the screw has been increased. To reduce the root diameter of the screw, the external diameter of the screw shaft should be also reduced. On the other hand, to increase the rotary driving force of the screw, the strength of the screw shaft should be increased. Thus, the strength of the screw shaft needs to be increased while the external diameter of the screw shaft is made smaller. To solve this problem, for example, the shaft disclosed in Patent Literature 3 can be used. In the shaft, a high strength spline is formed.
As described above, twin-screw extruders have been improved to increase the amount of materials to be processed per unit time. Recently, the root diameter of the screw or the external diameter of the screw shaft is approaching a limiting value in terms of design.